We continued to use transgenic shuttle vectors for the determination of in vivo mutant rates to learn about the role of somatic mutagenesis during malignant B-cell tumor development in mice. The shuttle vectors we are using are the phage l vector, lLIZ, and the plasmid vector, pUR288. We have backcrossed both shuttle vectors from strain C57BL/6 to strains BALB/c (C) and DBA/2N (D2). C and D2 are of interest to us because they display dramatic differences in the genetic susceptibility to Myc-induced B-cell neoplasia. The availability of lLIZ and pUR288 on three defined inbred backgrounds provides a unique experimental system in which the genetic susceptibility to B-cell neoplasia can be associated with the genetic control of mutagenesis in the B-cell compartment. Three major experiments were completed in FY 2001 with the help of the pUR288 assay. To evaluate somatic mutagenesis in early B progenitors, we prepared pre-B cells from fetal livers of C and D2 mice congenic for the mutagenesis shuttle vector, pUR288. We assessed both g-irradiation induced mutagenesis in differentiation arrested pre-B cells and endogenous mutagenesis in pre-B cells undergoing differentiation into surface IgM expressing immature B lymphocytes. We observed that pre-B cells are extremely susceptible to oxidative mutagenesis, that Bcl-2 is a potent inhibitor of pre-B mutagenesis, and that strain C is more susceptible to oxidative attack than strain D2. We concluded that the apparent preponderance of C to accumulate mutations in pre-B cells must be linked to the susceptibility of this strain to develop Myc-induced B-cell neoplasia. In the second experiment, we employed mice that harbored pUR288 and a low efficiency allele of the major isozyme of glucose-6-phosphate dehydrogenase (G6PD) to further our understanding of the interdependence of general metabolism, oxidative stress control, and somatic mutagenesis. We found that these mice exhibited a significant distortion of redox control, a considerable accumulation of promutagenic etheno DNA adducts, and a substantial elevation of somatic mutation rates in lacZ, the target and reporter gene of mutagenesis in the shuttle vector, pUR288. The mutation pattern in spleen and brain was dominated by illegitimate genetic recombinations. We concluded that G6PD is critical for limiting oxidative mutagenesis in the mouse. In the third experiment, we studied the mutator phenotype present in El/Myc-induced mouse Burkitt's lymphoma (mBL). We found that the mutant frequency in lacZ, the reporter gene of the pUR288 mutagenesis assay, was only moderately elevated in mBL, but that the mutational pattern in lacZ displayed a striking shift from point mutations to recombination (R) and deletion (D) mutations. Genomic instability in mBL seems thus characterized by a preponderance of R and D mutations in the context of near-background mutant frequencies. This finding supports the notion that deregulated Myc may act as a structural modifier of the genome.